Daniel James, the 23-year-old paralysed rugby player who became the youngest Briton to travel to the Dignitas assisted suicide clinic in Switzerland, was so determined to end his life that he told his parents he would move out of the family home and starve himself to death if they did not help him.

The first full police account of the case, which attracted international media attention as a rallying point for supporters and opponents of assisted suicide, reveals the pressure Daniel's parents, Mark and Julie James, were under, and the inhumanity of a law that placed them as suspects in a serious crime, subjected them to harrowing interrogation and compounded their grief as they waited for six weeks to hear if they would be prosecuted.

Daniel took his life at the Dignitas clinic on 12 September 2008. He had previously attempted suicide three times, twice by overdose and once by stabbing himself. Four times he was assessed by psychiatrists and each found him to be rational, not depressed and with the mental capacity to make decisions about his treatment.

Even so, his parents who accompanied him to Dignitas, and a family friend who paid £12,165 to charter a plane to fly the family to Switzerland were subjected to a lengthy and distressing police investigation as accessories to a crime under the Suicide Act 1961.

Daniel had been critically injured 15 months earlier, on 12 March 2007, during a rugby training session when a scrum in which he played hooker collapsed, leaving him with a severe spinal injury.

He was a third-year construction engineering student at Loughborough University and a keen sportsman who had played rugby for England at junior and university level and was expected to represent his country at senior level. Instead, he was left paralysed from the chest down, with little control or sensation in his hands, unable to wash or dress himself without assistance, and with no prospect of improvement.

His death split supporters of assisted suicide because of his youth and because his injuries, while extremely severe, were not terminal.

Giving evidence in public for the first time since the case to an inquiry into assisted dying chaired by Lord Falconer and organised by the think-tank, Demos, detectives Adrian Todd and Michelle Cook of West Mercia Police said they had been alerted to the family's plans to go to Dignitas by Worcestershire Social Services on 9 September, the day that the family left for Switzerland. Though they went immediately to the family home to warn Mark and Julie James, Daniel's parents, of the "legal implications of their intended actions" they were too late.

The Jameses were eventually interviewed, separately, for more than three hours each, on 6 October, after Daniel's body had been repatriated and his funeral held. Despite the efforts made by the detectives to deal sensitively with the case, their report makes clear the distressing impact the investigation had on all involved

Daniel made the first attempt on his life by swallowing an overdose while in the spinal injuries unit at Stoke Mandeville Hospital on 8 October 2007, seven months after his injury. Six weeks later he was discharged home and took a second overdose on 20 January 2008.

On that occasion he was rushed to hospital where a psychiatrist noted he was "very angry and extremly hopeless" and despondent that he had again failed in his suicide bid.

He refused any medical treatment but two days later was "calm, rational and co-operative" and stated that he wanted to die and if he did not do so from his physical problems he would continue to attempt suicide.

His parents, who up until then had done everything they could to dissauade him, told the psychiatrist they had come to accept his wish to die.

They had bought thousands of pounds worth of equipment for their home to help with his rehabilitation but he had shown no interest in using it. In March 2008, Daniel applied to go to Dignitas telling his mother that if he were rejected he intended to move out of the family home into assisted accommodation and starve himself, as "that was the only means of ending his life in a way that he was able to control".

His parents reluctantly helped him organise the trip to Dignitas and were with him at the clinic in Zurich on 12 September when he took the fatal dose of barbiturates, slipped into unconsciousness and died.

Mark and Julie James had been "very dignified and willing to co-operate" with the police investigation and provided a "very full account" of what had happend but they were "genuinely concerned they might go to prison," Detective Inspector Todd said. "I don't think until their solicitor pointed it out they were aware they had committed an offence," he added.

The family's GP had known about Daniel's plans, ordered psychiatric assessments and kept detailed records but had not alerted police on the grounds of protecting patient confidentiality. Clearer guidelines were needed for doctors, Mr Todd said.

The file on the case was sent to the Director of Public Prosecutions (DPP), Keir Starmer, on 23 October 2008 but he did not decide until six weeks later on 9 December that the Jamese would not be prosecuted. Though there was sufficient evidence to warrant prosecution, he concluded it would not be in the public interest.

The DPP subsequently published guidance in February 2010 which effectively decriminalised amateur assisted suicide by distinguishing between compassionate and malicious actions. But the policy had not changed how the police investigated cases, Detective Inspector Todd said.

Only one out of 42 police forces had a policy on dealing with assisted suicide and though a national policy was being developed he would have to follow the same investigative procedure in any similar case today, he said. "The delay in informing the family they would not be prosecuterd did not help their situation at a very difficult time in their life," he added.

An inquest was subsequently held in Worcestershire where it was deemed that Daniel took his own life.